A MICROSCOPICAL STUDY OF THE DEVELOPMENT OF A SPORE‐LIKE STAGE IN THE LIFE CYCLE OF THE ROOT‐NODULE ENDOPHYTE OF ALNUS GLUTINOSA (L.) GAERTN.

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Abstract
SUMMARY: A light‐ and electron‐microscopical study of the root‐nodule endophyte of Alnus gluttinosa (L.) Gaertn. was carried out to investigate the development of a spore‐like stage, here called the granule, in the life cycle of the endophyte. Comparison of granule‐rich and granule‐free root nodules showed that granule formation takes place via local transverse growth of thick endo‐phytic hyphae, giving rise to multicellular‘granulated bodies’differing in shape and size. Subsequently, the cells of these granulated bodies are transformed into granules by cell separation and ultrastructural changes, the most striking of which are cell‐wall thickening, reduction of the number of mesosomes, and increased density of the cytoplasm. Granule development takes place both intracellularly and intercellularly. Intracellularly produced granules are eventually liberated by the death of the host cell. Mature granules show a strong resemblance to spores of free‐living actinomycetes in their ultrastructure and behaviour.It is concluded that, in view of the terminology currently used in the description of members of the Actinomycetales, the term granule should be replaced by spore and thus the term granulated body by sporogenous body.